The present invention relates to a nuclear reactor fuel assembly, whereof the rods are fixed to an end piece and in which the system of grids is kept supported against one of the end pieces. The invention relates to nuclear reactor fuel assemblies and particularly those used for water-cooled reactors.
Each assembly of this type extends over a length of several metres between a lower inner plate and an upper inner plate belonging to the equipment within the reactor vessel. It rests on the lower inner plate and is positioned by centering pins forming an integral part of the two aforementioned plates connected to the reactor vessel. It comprises a rigid mechanical frame formed by two end pieces, namely the upper and lower end pieces, provided with openings permitting the passage of the cooling water circulating from bottom to top, as well as a certain number of connecting rods ensuring the stability of the system by being fixed to the said end pieces. The supporting and spacing of the fuel rods is brought about along each assembly, by a certain number of supporting grids connected to the connecting rods defining cavities having a square cross-section.
Furthermore, in order to permit the expansion in operation of the fuel rods, it is necessary to leave a clearance between the rods and at least one of the end pieces. In addition, as the assembly is exposed to a flow of cooling water, which circulates from bottom to top in the reactor core, it is necessary to avoid, as a function of the type of assembly, either the flying off of said assembly, or the flying off of the grids and the fuel rods under the effect of the cooling water circulation.
Numerous embodiments of nuclear reactor fuel assemblies are known providing a solution to the aforementioned problems, for example, those described in French Pat. Nos. 1,536,257 and 2,049,108 granted to the Westinghouse Electric Corporation. French Pat. No. 1,536,257 relates to the design of a framework for a fuel assembly equipped with a group of fuel rods sliding in the connecting rods. The latter are made from steel and the supporting grids are welded thereto and are made from Inconel. The fuel rods are supported by springs and bosses stamped into the plates of the grids.
French Pat. No. 2,049,108 relates to the design of an assembly, whose connecting rods are made from Zircaloy, whilst the grids are made from Inconel. Due to the metallurgical incompatibility of these two metals, the connection between the grid and the connecting rod takes place by the interposing of a stainless steel sleeve hard soldered to the grid plates, which is then mechanically deformed together with the connecting rod.
According to another known construction, the spacing grids slide freely on the connecting rods. Only the lower grid is held on the lower end plate by a fly-off-prevention sleeve. In this case, the spacing grids are made from a single material, preferably Inconel, which ensures an adequate fastening to the fuel rods for the different grids to be supported and retained by means of the said fuel rods.
The fuel assembly according to the invention has homogeneous grids without joined springs and made from zircaloy, whose neutron absorption power is particularly low.
Attempts were firstly made to produce spacing grids entirely from Zircaloy by cutting in the walls, springs able to exert an adequate fixing force of the fuel rods to ensure the secure fixing between grids and fuel rods throughout the life of the assembly. However, tests have shown that this result could not be achieved due to the relaxation of Zircaloy under irradiation. At the end of a certain time, the fixing effect of the springs to the fuel rods slackens and it is no longer possible to ensure the fastening between the grids and the rods.